In general, a hydraulic excavator as a representative example of a working machine used at a construction site and the like, is provided with a vehicle body comprising an automotive lower traveling structure and an upper revolving structure that is mounted on the lower traveling structure to be able to revolve thereon. A working mechanism for performing an operation such as an excavating operation and the like is mounted on the front side of the upper revolving structure.
On the other hand, there is known a hybrid-type working machine using both an engine and an electric motor together, as a working machine such as a hydraulic excavator and the like. This hybrid-type working machine includes an engine, an assist generator motor that is driven by the engine to generate power or assists in a driving force of the engine by supply of power thereto from an electricity storage device, the electricity storage device that stores the power generated by the assist generator motor or discharges the power therein, and an inverter for controlling an operation of the assist generator motor (Patent Document 1).